Not applicable.
Not applicable.
This invention relates to hydrogen gas production from a gas containing methane, and more particularly to the joint production of hydrogen gas and syngas containing carbon monoxide gas and hydrogen gas.
Currently there are four basic technologies available for producing hydrogen and syngas from natural gas feedstock: (1) steam-methane reforming (SMR); (2) secondary reforming with oxygen (SMR/O2); (3) autothermal reforming (ATR); and thermal partial oxidation (POx). Each of these four technologies produces syngas with inherently different H2/CO ratios. The range of these ratios can be adjusted by varying the extent to which CO2 is recycled. The ratio is inversely proportional to the percentage of CO2 recycled. That is, the highest H2/CO ratio is obtained when no CO2 is recycled, and the lowest H2/CO ratio is obtained when all CO2 is recycled. Table 1 summarizes H2/CO ratios range for different technologies:
Syngas can be obtained at a ratio below the ratios indicated in Table 1 by further processing the reforming effluent in additional gas separation equipment, such as a membrane or PSA apparatus. However, additional gas separation equipment is expensive to purchase and maintain. There have been several attempts made in the past to achieve H2/CO ratios of syngas below those reached at full CO2 recycle (i.e., ratios of 2.5 and lower) directly from the reforming stage without using gas separation equipment.
For example, GB Patent No. 1,185,450 discloses a method wherein additional CO2 is fed to the reformer feed from an outside source to move the following reforming equilibrium:
CO+H2O⇄CO2+H2xe2x80x83xe2x80x83Equation I
towards more CO production and less H2 production (i.e., towards the left). Although this can be an effective and economical method, it requires a ready source of inexpensive gaseous CO2, which is often not available for CO-rich syngas production. CO2 is generally available from ammonia and conventional hydrogen plants with methanators and CO2 removal systems. However, for currently designed hydrogen plants providing high purity hydrogen supply, there is no CO2 available since they typically use PSA technology and generally eliminate the CO2 removal system and methanator. High purity hydrogen product is the preferred choice of supply in most cases and hydrogen plants designed with PSA are taking more and more market share.
Since there is no CO2 source from those types of plants, the only option to produce CO-rich gas from the reformer effluent side stream is to use costly gas separation equipment. Additionally, extra hydrogen recovered from the side stream to adjust H2/CO ratio in the syngas is often used only as a fuel.
Several patents propose to shift the reforming equilibrium towards CO (i.e., shift Equation I, above, to the left) by making changes to the catalyst system in order to achieve low H2/CO ratios in the syngas. For example, U.S. Pat. No. 5,336,655 discloses a precious metal-based catalyst system said to be capable of achieving a H2/CO ratio as low as 0.85. Data presented in the patent are based on laboratory data at 100 hours of operation. There is no information about operating characteristics of the disclosed catalyst system in an industrial unit, such as the expected life, resistance to poisons, requirements to S/C ratio, etc. Also, carbon formation of about 0.5 mg/g catalyst reported after 100 hours of operation seems to be excessive.
Dibbern et al., xe2x80x9cMake low H2/CO syngas using sulfur passivated reforming,xe2x80x9d Hydrocarbon Processing 71-74 (January 1986), discloses a process of producing low H2/CO syngas product using both imported CO2 and partially poisoned reforming catalyst. The article discloses carbon-free operation of the reformer at significantly reduced steam/carbon ratios down to 0.9. Although the economics of this process demonstrated for the monotube pilot plant unit look attractive, sustained operation of a unit employing the disclosed technology strongly depends on controlled sulfur passivation of the catalyst. The extent of partial catalyst passivation performed by carefully controlled sulfur injection in the reformer feed is a complex variable which is difficult to maintain for a multi-tube reformer without a complicated real-time on-line catalyst evaluation system.
U.S. Pat. No. 5,496,530 discloses another process for preparing CO-rich syngas from a gas mixture of H2 and CO2 over a conversion catalyst. The process occurs adiabatically so that the following exothermic methane-producing (methanation) reaction:
CO2+4H2⇄CH4+2H2Oxe2x80x83xe2x80x83Equation II
provides necessary heat for the following endothermic CO producing reaction:
CO2+H2⇄CO+H2Oxe2x80x83xe2x80x83Equation III
The process is characterized by very low capital costs, but requires both H2 and CO2 sources to be available. The lowest H2/CO ratio disclosed is 1.5. Significant fractions of methane and CO2 are present in the final syngas product.
EP 0816290 discloses a process of simultaneous production of pure CO and hydrogen or ammonia, wherein a side stream is removed from a secondary reformer effluent of a conventional steam-methane reforming plant, the side stream is cooled to condense out water vapor, and the gas components of the side stream are separated from CO in a gas separation train (CO2 removal, PSA, TSA, cold box), compressed, preheated and recombined into CO-lean syngas stream feeding the CO shift. The conventional plant includes primary and secondary reforming, CO shift, CO2 removal system and methanator used to produce hydrogen or ammonia syngas. The process enjoys cost benefits of integrating co-production of CO with main H2 or NH3 production, but cannot be cost effectively used for the co-production of CO-lean synthesis gas, since that requires mixing of pure CO with CO-rich syngas.
All references cited herein are incorporated herein by reference in their entireties.
The invention provides a process and an apparatus for simultaneously producing a syngas product having a H2/CO ratio of less than 2.5 and a hydrogen gas product, wherein the process comprises increasing the amount of carbon dioxide being fed to a secondary reformer with carbon dioxide extracted from: (a) an effluent from a primary reformer and (b) an effluent from the secondary reformer.
A preferred process of the invention comprises: (a) diverting a portion of an effluent from a primary reformer to provide a side stream separate from a primary stream of the effluent; (b) reacting water and carbon monoxide in the primary stream to provide an enriched primary stream having increased amounts of carbon dioxide and hydrogen therein; (c) extracting carbon dioxide from the enriched primary stream to provide a hydrogen enriched primary stream and primary extracted carbon dioxide; (d) processing the hydrogen enriched primary stream to provide the hydrogen gas product; (e) feeding the side stream to a secondary reformer; (f) cooling a secondary reformer effluent to provide a cooled side stream; (g) extracting carbon dioxide from the cooled side stream to provide the syngas product and secondary extracted carbon dioxide; and (h) combining the primary and secondary extracted carbon dioxide with the side stream upstream of the secondary reformer.
The apparatus of the invention preferably comprises: (a) a primary reform; (b) a pressure swing adsorption vessel in fluid communication with said primary reformer and adapted to receive a primary stream of said effluent from said primary reformer; (c) a secondary reformer in fluid communication with said primary reformer and adapted to receive a secondary stream of said effluent from said primary reformer; (d) a primary carbon dioxide absorber in fluid communication with said primary reformer and adapted to absorb gaseous carbon dioxide from said primary stream; (e) a secondary carbon dioxide absorber in fluid communication with said secondary reformer and adapted to absorb gaseous carbon dioxide from said side stream; and (f) a carbon dioxide stripper in fluid communication with said primary carbon dioxide absorber, said secondary carbon dioxide absorber and said secondary reformer, wherein said carbon dioxide stripper is adapted to extract carbon dioxide from said primary carbon dioxide absorber and from said secondary carbon dioxide absorber, so as to recycle carbon dioxide to said secondary reformer.